1. Field of the Invention
This invention relates to metal compositions and more particularly, to whisker-reinforced metals (WMR) which are suitable for use in aircrafts, space crafts, automobiles, sports goods and the like. Also, it relates to free-cutting metal compositions which are suitably machined by cuttings such as lathing, boring, gear cutting and broaching or by grindings using grinding wheels.
2. Description of the Prior Art
Since whiskers have generally a very small number of dislocations with an attendant advantage that the strength is close to an ideal value of the crystals, they have been used in combination with various metals to improve the strength and the modulus of elasticity. Typical whiskers known in the art include those of .beta.-SiC, .alpha.-SiC, .alpha.-Si.sub.3 N.sub.4,, graphite (C), potassium titanate (K.sub.2 O.multidot.6TiO.sub.2), Al.sub.2 O.sub.3, Cu, Fe, W and the like.
When metals are reinforced with these whiskers, not only the strength and the modulus of elasticity are improved, but also high temperature strength is remarkably improved along with an improvement of wear resistance. In addition, as is different from the case of FRM where continuous fibers are used, the whisker-reinforced metals have the advantage that they can be fabricated such as by rolling, extrusion, forging or the like.
On the other hand, for the ease in machining and the high machining accuracy, there is a demand of metal materials which have good free-cutting properties. To this end, attempts have been made wherein various elements or components are added to metals. Some metal compositions have now been put into practice. Known additive components include, for example, elements such as Cu, Pb, S, Mn, Si, C, P, N, Se, Te, Bi and the like, inorganic fillers such as calcium silicate, mica, talc, asbestos, mineral fibers and the like, and inorganic whiskers such as of potassium titanate. For imparting good free-cutting properties, these components have to be undesirably compounded in large amounts.
In particular, the use of known whiskers for reinforcement of metals is not always favorable. The known whiskers are in the form of simple needle-like fibers. When these whiskers are mixed, for example, with a metal in the form of powder or melt and pressure is imposed on the mixture such as by extrusion, the whiskers are apt to be aligned or oriented in one direction along which the pressure has been imposed. This will cause the strength of the mixture to be anisotropic. High strength is obtained along the direction of the alignment, but the effect of the reinforcement considerably decreases along directions which are deviated even only slightly from the alignment direction.
For obtaining high strength, it is general to formulate 15% by volume to 30% by volume or over of the whiskers. The formulation of such a large amount of the whiskers makes composite materials which are too hard from the standpoint of cutting or grinding operations. Thus, the composite materials are difficult to machine.
Moreover, known whiskers are complicated in manufacture process with a poor yield and are thus expensive.